The present invention relates to storage medium of the type which move relative to a transducer. More specifically, the present invention relates to detection of asperities in a recording surface of the storage medium.
Storage mediums are used to store information for later retrieval. For example, magnetic discs are a type of storage medium in which a surface of the disc carries magnetically encoded information. The disc is rotated at a high speed and a magnetic transducer (a head) is positioned to "fly" just over the surface of the rotating disc. The head is used to write information onto the surface of the disc by changing the magnetization across the surface of the disc. Information is then read back from the disc by the head which detects the changes in the magnetization carried on the disc surface. One type of head is a thin film inductive head which can be used for both reading and writing magnetically encoded information. Another type of head is a magnetoresistive sensor which has a resistance which changes in response to an external magnetic field. Binary information is stored on the disc surface as a series of flips in the direction of magnetization across the surface.
The technology in this area has been moving toward ever increasing levels of storage density. Increased storage density is achieved by placing more magnetically encoded information in a given area on the disc surface. In order for the transducing head to properly record and read back this information, the head must be placed extremely close the disc surface. This is achieved by allowing the head to "fly" over the disc surface as the disc rotates at high speed. The flying is due to the hydrodynamic properties of the air passing between the slider and the disc.
The distance between the head/slider assembly and the disc surface is extremely small. A minor asperity in the surface of the disc can cause catastrophic damage to the head, and/or disc surface, leading to failure of the storage system. Thus, tests have been developed which detect asperities in the surface of the magnetic disc. Typically, the prior art has detected asperities by monitoring vibrations induced in the head due to an impact or an interference between the asperity and the slider. One prior art technique uses a piezoelectric element which is attached to the slider. When the slider contacts the medium, the head vibrates causing the piezoelectric element to generate an AC signal. The time at which the AC signal is detected can be used to identify the location of the asperity on the disc surface because at that moment the asperity will have just passed the location of the piezoelectric element. Further, the size of the asperity can be determined by noting the amplitude of the AC signal.
Another technique for identifying asperities in a disc surface is described in U.S. Pat. No. 4,881,136, issued Nov. 14, 1989 to Shiraishi et al. entitled "METHOD AND APPARATUS FOR DETECTING MINUTE DEFECTS ON MAGNETIC DISC BY MONITORING BOTH AMPLITUDE DECREASE AND PHASE SHIFT OF A REPRODUCED SIGNAL". A somewhat related technique is described in U.S. Pat. No. 4,942,609, issued Jul. 17, 1990 to Meyer entitled "ELECTRONIC FLYING INTEGRITY TESTER FOR DISC DRIVE". Both of these systems monitor the output of a transducing head. As the transducing head passes over the magnetized disc surface, the output from the head changes in response to stored information. However, if the head passes over or impacts an asperity, the output signal from the transducing head will change substantially. The amplitude of the output signal will change as the distance between the head and the disc surface changes. Additionally, a sinusoidal signal will be superimposed on the output signal as the head oscillates or rings after the interference with the asperity. Commercial systems are available from Phase Metrics of San Diego, Calif. and Hitachi of Tokyo, Japan. In all of these techniques, if the asperity is larger than a desired threshold, the test, which is referred to as a glide test, is considered a failure. Further, the significance of accurate glide tests is becoming increasingly important as the flying height becomes less.
Prior art glide testers suffer from a number of problems. In order to detect small defects, the glide tester must have a good response to high frequency signals. However, this response is difficult to control during manufacturing of the tester. Additionally, there is a lack of linearity in the size of the response and the size of the asperity. This lack of linearity makes it difficult to determine the threshold for detecting a failure. The testers typically use a calibration disc carrying calibration asperities which are small bumps ranging from 4 to 12 mils. However, bumps with differing aspect ratios cause the sensor to respond in different ways making the calibration curve generated from such a calibration disc valid for only a certain type of defect. Furthermore, the sensitivity of the glide tester varies during the course of testing and depends upon the fly height, head contamination and damage, mechanical damage, etc. Additionally, in a prior art glide tester, a damaged sensor may go undetected. This will cause all subsequent discs to pass the glide test, regardless of their condition.